FIG. 1 illustrates Radio Link Monitoring (RLM) and Radio Link Failure (RLF) procedure in LTE. The purpose of the Radio Link Monitoring (RLM) function in the user equipment (UE) is to monitor the downlink radio link quality of the serving cell in RRC_CONNECTED state. Monitoring is based on the Cell-Specific Reference Signals (CRS). This in turn enables the UE when in RRC_CONNECTED state to determine whether it is in-sync or out-of-sync with respect to its serving cell as described in TS 36.213 V14.3.0, Section 4.2.1.
The UE's estimation of the downlink radio link quality is compared with out-of-sync (OOS) and in-sync (IS) thresholds, Qout and Qin for the purpose of RLM. These thresholds are expressed in terms of the Block Error Rate (BLER) of a hypothetical Physical Downlink Control Channel (PDCCH) transmission from the serving cell. Specifically, Qout corresponds to a 10% BLER while Qin corresponds to a 2% BLER. The same threshold levels are applicable with and without Discontinuous Reception (DRX).
The mapping between the CRS based downlink quality and the hypothetical PDCCH BLER is up to the UE implementation. However, the performance is verified by conformance tests defined for various environments, as described in 3GPP TS 36.521 V14.3.0 and 3GPP TS 36.133 V14.2.0.
The RLF is triggered when timer T310 expires, Random access problems have occurred or too many Radio Link Control (RLC) retransmissions have occurred.
Relating to RLM in NR, it has been agreed that for connected mode a UE declares RLF upon timer expiry due to DL OOS detection, random access procedure failure detection, and RLC failure detection.
The physical layer performs out of sync/in sync indications and RRC declares RLF. It has also been agreed that RLF is triggered based on RLC max number of retransmission reached for a single leg. One of the remaining open issues was whether there will be per cell IS/OOS indications or not. However, at least periodic IS/OOS indications are planned. Additionally, it is expected that the LTE model for RLM/RLF should also be adopted as baseline for NR.
In NR, in order to improve coverage and increase data rate, beamforming is widely used. With beamforming, network can transmit user specific data via narrow beams which can improve Signal-to-Interference-plus-Noise Ratio (SINR). One issue with beam-based transmission is that since beams can provide quite narrow coverage, it is possible that suddenly a UE is out of the coverage of the beam. If that occurs, the network would not be able to efficiently schedule data to that UE and/or the UE would not be monitoring the right beam (or beam link pair) used by the network to transmit a control channel (like PDCCH) and the UE would not be able to detect scheduled information. That problem is typically called “beam failure”.
3GPP has acknowledged the importance of that problem and started to discuss for 5G a procedure called beam recovery upon the detection of a beam failure for RRC_CONNECTED UEs. In beam recovery, an RRC_CONNECTED UE would perform measurements associated to the quality of the serving link and, if that quality goes below a given threshold, the UE would perform beam recovery. The procedure aims to solve the situation where the TX and/or RX beams of the NR radio base station (e.g. a gNodeB) and the UE have become misaligned, but where there are additional beams that could be used to maintain the connection between the gNodeB and the UE.
The beam failure recovery procedure includes the following aspects:                Beam failure detection: Here the UE monitors a certain periodic reference signal (RS) to estimate the quality of the serving link. Once the quality of that link falls below a certain threshold, the UE initiates beam recovery.        New candidate beam identification: Once beam failure has been detected, the        
UE tries to identify a new beam that would provide adequate quality. The UE then searches for a specific Reference Signal (RS), which is transmitted from the same node, but in difference candidate beams. During this search procedure, the UE may also change its RX beam.                Beam failure recovery request: Once a new candidate beam has been found, the UE transmits an UL signal using certain UL resources. The gNodeB is prepared to receive the UL signal in these UL resources, and can determine which candidate beam the UE selected based on the receive UL signal.        Beam failure recovery response: When the gNodeB has received the beam failure recovery request, it sends a DL response to indicate to the UE that it received the request, using the knowledge of the new beam. UE monitors gNB response for beam failure recovery request. Once the UE has successfully received the response, the beam recovery is complete.        
Thus, there have been significant discussions about beam recovery (BR) for NR, and the associated monitoring procedure. In RAN1, it was agreed that a beam failure event occurs when the quality of beam pair link(s) of an associated control channel falls low enough. As the user equipment (UE) is monitoring downlink (DL) quality, such a beam link monitoring (BLM) would be equivalent to monitor the quality of the PDCCH. According to RAN1, such IS/OOS indication is at least periodical and such indicator is irrespective of number of beams in a cell. Further, RAN1 has agreed that there will be periodic reference signals (RS), synchronization signal block (SSB) or Channel State Information-Reference Signal (CSI-RS) that represents the PDCCH quality. Also, that there can be aperiodic indications to assist Radio Link Failure (RLF) if the monitored RS is same for Beam Link Failure (BLF) and RLF. Any such assistance implies that there is a connection between beam link failure and radio link failure.
The beam recovery procedure has been designed to recover a lost beam, using another beam from the same cell. Since the UE re-establishes connection with the same cell, the overhead of the procedure is small: e.g., the UE is not required to perform cell reselection, or acquire system information. In contrast, if the UE triggers RLF, it may have to reselect a new cell, and acquire system information, with the associated overhead and delay.